A conventional shadow-mask-type CRT comprises an evacuated envelope having therein a viewing screen comprising an array of phosphor elements of three different emission colors arranged in a cyclic order, means for producing three convergent electron beams directed towards the screen, and a color selection structure or shadow mask comprising a thin multi-apertured sheet of metal precisely disposed between the screen and the beam-producing means. The apertured metal sheet shadows the screen, and the differences in convergence angles permit the transmitted portions of each beam to selectively excite phosphor elements of the desired emission color. A matrix of light-absorptive material surrounds the phosphor elements.
U.S. Pat. No. 3,475,169, issued to H. G. Lange on Oct. 28, 1969, discloses a process for electrophotographically screening color cathode-ray tubes. The inner surface of the faceplate of the CRT is coated with a volatilizable conductive material and then overcoated with a layer of volatilizable photoconductive material. The photoconductive layer is then uniformly charged, selectively exposed with light through the shadow mask to establish a latent charge image, and developed using a high molecular weight carrier liquid. The carrier liquid bears, in suspension, a quantity of phosphor particles of a given emissive color that are selectively deposited onto suitably charged areas of the photoconductive layer, to develop the latent image. The charging, exposing and deposition process is repeated for each of the three color-emissive phosphors of the screen. An improvement in electrophotographic screening is described in U.S. Pat. No. 4,448,866, issued to H. G. Olieslagers et al. on May 15, 1984. In that patent, phosphor particle adhesion is said to be increased by uniformly exposing, with light, the portions of the photoconductive layer lying between the deposited pattern of phosphor particles after each deposition step, so as to reduce or discharge any residual charge and to permit a more uniform recharging of the photoconductor for subsequent depositions.
The two above cited patents disclose an electrophotographic process that is, in essence, a wet process. A drawback of the wet process is that it may not be capable of meeting the higher resolution demands of the next generation of entertainment devices and the even higher resolution requirements for monitors, work stations and applications requiring color alpha-numeric text. Additionally, the wet process (including matrix processing) requires a large number of major processing steps, necessitates extensive plumbing and the use of clean water, requires phosphor salvage and reclamation, and utilizes large quantities of electrical energy for exposing and drying the phosphor materials.
U.S. Pat. Nos. 4,921,727 and 4,921,767, issued May 1, 1990 to P. Datta et al., and U.S. patent application Ser. No. 287,356; 287,358 and 287,355, by P. Datta et al., filed on Dec. 21, 1988, describe an improved process for manufacturing CRT screen assemblies using triboelectrically charged dry-powdered screen structure materials, and surface-treated phosphor particles having a coupling agent thereon to control the triboelectric charging characteristics of the phosphor particles. During the manufacturing process, the surface-treated screen structure materials are electrostatically attracted to the photoconductive layer on the faceplate, and the attractive force is a function of the magnitude of the triboelectric charge on the screen structure materials. Thermal bonding has been utilized to affix the relatively loosely bonded surface-treated materials to the photoconductive layer; however, thermal bonding occasionally causes cracks in the photoconductive layer, which becomes detached during a subsequent filming step in the manufacturing process. Additionally, it is desirable to eliminate the fusable thermoplastic phosphor coating that is used with some of the above-identified triboelectrical processes since such coatings add additional organic materials which can negatively affect phosphor emission efficiency. It has been determined that an alternative method of dry filming is thus desirable to increase phosphor efficiency, screen uniformity and adherency, while preventing the loss of screen assemblies during the manufacturing process due to cracked or detached photoconductive layers.